Exhaust Backpressure ???

What does Exhaust Backpressure mean to you ????

1- actual total pressure in PSI in exhaust system

2- the pressure influence of returning (Back) positive acoustical wave

3- other (state description)

I usually think number 1.

My thoughts:

I think of it as the average (not instantaneous) pressure in the exhaust measured just before the muffler(s) with a mechanical pressure gage, like a fuel pressure gage. That would be closest to choice 1.

There are lots of pressure variations upstream in the headers and collectors as in your choice 2. Measuring these variations would require real time gaging with pretty quick response time.

In my somewhat convoluted mind, "back pressure" relates to the overall mechanical flow of the gasses, not the acoustical tuning pressures, even though there are certainly "backpressure" pulses, and well as "negativepressure" pulses. How about "compressions" and "rarefractions" in the flow?

I guess the terminology could get confusing much like Dynamic Compression Ratio which is often just a fixed calculated value for a given engine combination, and Effective Compression Ratio which is what the engine sees during opoeration and varies a lot, probably with VE (which means Volumetic Efficiency to some and Valve Events to others.:) )

Larry, what's your answer?

I'll let you know after reading my book.

For now, all I can say is that it's a nasty term and I can't find any
benefit of having backpressure in the exhaust system at this point
in reading (chapter 4).

My non-scientific response would be the amount of "fluid pressure"
built up between the exhaust valve and restriction point(s) as the
piston tries to force more charge out of the cylinder at higher RPM.

The restrictions may be pipe design (shape;volume (length & diameter); pipe material; etc), catalytic flow, muffler flow and atmospheric pressure.

Larry, what's your answer?
=======================

i just wanted to see what majority thought meaning was to them

number 1 will probably be the majority

-----------------------------------

like you said about DCR terms can cause a little confusion

same with "Back Pressure"
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i look at Back Pressure as total exhaust system pressure in psi
and what psi might exists near or at exhaust valve, especially during overlap period.

there is no way an increase exhaust back pressure in psi
during overlap period or exhaust stroke, that will help HP/Torque

more exhaust psi should cause more exhaust dilution of mixture during overlap , along with less dense mixture from increased heat, and reduced intake port velocity along with reduced ram-effect near intake valve closing point (Ve Loss)

if you flow 1 5/8 headers -vs- 1 3/4 -vs- 1 7/8 -vs- 2 inch on FlowBench there isn't but a few CFM differences between all these sizes ..yet substantial HP/TQ differences

if you flow test a 1 3/4 dia header , one 20 inches long -vs- another at 34 inches long ...on the Flowbench there isn't much CFM difference ..but in live engine there can be huge differences at certain RPMs

so when changing/testing header pipe diameters and lengths
you are changing wave tuning

what you misinterupt as "Back Pressure" changing is really wave tuning changes..and not changes in system psi

some racer might say, i made a pass without mufflers with just open headers and slowed down ..i guess the engine needed the "back pressure" ?

or another racer might say , my car went faster with collector add on length..i guess it needed the "backpressure"

reality=> wave tuning was cause of effects, not backpressure

in all research papers i've read, back pressure always hurts HP/Torque

^ I get in debates about that all the time hence the creation of
the "Backpressure vs. Resonance Tuning" thread I started a couple
of months ago.

Now with all the help from this forum and the book, I'm going to
annihilate the next person that tries to tell me backpressure is beneficial! :)

From the perspective of the OEMs, backpressure is entirely #1. We measure backpressure at a tap just in front of the catalyst. Some groups meausure static backpressure only and some like static + dynamic using a Kiel probe. Either way, they like to see peak average pressure at WOT. The pulse tuning you refer to isn't really brought up that I've ever heard for normal cars.

I messed around with some sampling hardware and transducers years ago that were just about fast enough to see the individual pulses. There wasn't any real need for that type of resolution at the OEM level, so we stopped pursuing it.

This is a whole different class.

Zero, :D


In some cases back pressure is beneficial. Keep reading. Cut all of the exhaust off a 5.3L truck and they don't run any faster. Those motors are all ready lacking in the low-end department.

www.pacific-audio.com/performance/discussion.html

Go make a post here or search.

Ben T.

i"m going to disagree...but I'm not going to debate because my terminology is not up to par with many who post here.

The "backpressure" everyone refers to is misunderstood.

The engine requires some sort of medium to channel the 'fluid' out
of the chamber.

I'll let the big boys debate for me. Any scientific article that I have
read does not favor any sort of backpressure to make power/torque.

I just read that link...and several posts.

It's incredibly wrong.

"Needing backpressure to prevent air from travelling back into
the exhaust"?

Ummm....sure :rolleyes:

Oscar Wilde's quote.

Oscar Wilde the poet?

What does he know about engine technology?

Originally posted by Zero_to_69
Oscar Wilde the poet?

What does he know about engine technology?


LOL! Like many of us Old Farts, Oscar finally realized that sometimes with age comes wisdom.

The downside to that is: "Sometimes Age shows up alone."

"Sometimes Age shows up alone."

This happens more than I'd expect! Sometimes people's experiences are more limited than they guess and all this supposed "experience" is basically the same things happening over and over again so it becomes the truth to this particular old codger. In turn then they become close minded and jump to conclusions. Even worse are those with NO experience that have strong opinions however!

Originally posted by racer7088
"Sometimes Age shows up alone."

This happens more than I'd expect! Sometimes people's experiences are more limited than they guess and all this supposed "experience" is basically the same things happening over and over again so it becomes the truth to this particular old codger. In turn then they become close minded and jump to conclusions.

Should I resemble that remark? :)

Even worse are those with NO experience that have strong opinions however!

That happens about as much as I expect!



Experience without understanding may be worse than understanding without experience.

Suzuki employs a variable pressure system that works quite well on their GSXR1000.

http://www.superbikes.co.za/images/suzuki/GSXR1000_Engine2.jpg

http://www.superbikes.co.za/images/suzuki/GSXR1000_Exhaust1.jpg



Good day,
Mike

NO I wasn't referring to you OLDSSTROKER !

Originally posted by Mad Machinist
Suzuki employs a variable pressure system that works quite well on their GSXR1000.

If backpressure is induced, it is done, to achieve a NET gain. Generation of a net gain, by tailoring exhaust wave timing, to their advantage, in spite of, and even though there is a slight giveback, with any resulting extra backpressure.

I am somewhat a novice, but could the solution to backpressure end the great header debates that rage on in LT1 Tech? I have spent so much time trying to sort out the Shorty/Mid/LT information that I am confused about what is better for what. Is the LT the true almighty? Or does the Shorty rule the top RPMs. I sure as hell dont know, but does backpressure answer these questions? Also that is why I-pipe diameters are regulated, such as 3in aftermarket b/c a 5 inch one would hurt most 350 applications more than benefit from the extra flow?

-Dustin-

Originally posted by Bersaglieri
I am somewhat a novice, but could the solution to backpressure end the great header debates that rage on in LT1 Tech? I have spent so much time trying to sort out the Shorty/Mid/LT information that I am confused about what is better for what. Is the LT the true almighty? Or does the Shorty rule the top RPMs. I sure as hell dont know, but does backpressure answer these questions? Also that is why I-pipe diameters are regulated, such as 3in aftermarket b/c a 5 inch one would hurt most 350 applications more than benefit from the extra flow?

-Dustin-

Here are two recent threads that might help:

http://web.camaross.com/forums/showthread.php?s=&threadid=215779

http://web.camaross.com/forums/showthread.php?
s=&threadid=232645


The first few pages of the "Myth" thread have some good, and some confusing information. Melkor has some good words.

The second thread discusses flow and pipe sizes.

If you read thru these you might get some of the answers you seek...or maybe just get more confused. ;)

i think the easiest answer to the backpressure/no back pressure debate would be that no back presssure, ex: a pipe which would be 8" large would allow the air to eddy and not flow at the highest rate. a certain size pipe, which would be determined by the amount of air which is flowing from the engine would be one which would allow the exhaust gases to travel fastest thorugh the exhaust piping without allowing the the air to eddy back and make an attempt and succeed in making it back to the engine (will not make it back, but will be pressuring the gases behind, therefore limiting exhaust flow). the correct piping size will still have the gases attempting to make it back, but it will still be small enough that it cannot swirl and slow down the gases behind it.

therefore backpressure in an exhaust would be gases flowing through the piping that slow down the gases behind it.
optimal backpressure is exhaust flow in which the gases flowing do not cause a signifigant loss of speed to the gases behind it

sorry about my book, this is just what i learned working on turbo cars, which require no backpressure as torque is not the main concern, and just applied to the way a n/a vehicle creates power

Originally posted by romoranger
this is just what i learned working on turbo cars, which require no backpressure as torque is not the main concern,

You must mean no backpressure after the turbine, right? At turbine inlet there is probably significant backpressure, which the engine feels. Like MaxRaceSoftware said above, it depends on where you look at the backpressure.

FWIW, some car engines are turbocharged for the very reason of getting high torque at low rpm and holding the torque curve flat for maybe 2500-3000 rpm. An example would be Audi TC'd engines. WRX seems to get a torque peak around 4000 then drop off and flatten out. It appears to be an engine TC'd for mid-range torque.

If it seems like I'm picking you, that's not the intent. Just disagreeing a little. No offense intended. :)

Will hot burnt exhaust gases "Burn a Second Time " ????

Why would you want hot burnt exhaust gases back inside your engine's cylinders ???

Originally posted by MaxRaceSoftware
Will hot burnt exhaust gases "Burn a Second Time " ????

Why would you want hot burnt exhaust gases back inside your engine's cylinders ???

I wouldn't want it of course. But if there was a NET gain in fwhp from adding a turbocharger and accepting some backpressure resulting from the exhaust gases drivng the turbine, I would accept that.

If there's no turbo in the system, get those nasty hot gasses the hell out of there. :)

If you want to burn the exhaust a second time (assuming no cats which kinda do that), how about an afterburner? :)

Larry, how much "extractor effect" do you think Cup cars get from placement of the flat exhaust outlet in the rocker area? It would seem the aero guys could create a low pressure area there and maybe actually measure the depression in the pipes in a wind tunnel. It might be significant at 190 mph.

no problem at all oldsstroker, i am not really implying with stock power curves, which is something i probably should have indicated. and yes you are right, i had meant no pressure after the turbine but forgot to say that, i got a wicked concussion the other day so the fact that i am putting out somewaht complete thoughts is an accomplishment in itself today :D
but you are right about the high-torque at a low rpm, i completely forgot about diesels and the like. once again i should have specified for cars that are usually involved in some kind of racing, (excluding auto-x as torque is of a high concern) my head hurts so i will step down from lowly podium before i say something else dumb :D

Inert gases can't burn, they displace fresh intake charge :D

Do I get a cookie Larry/Bret/Jon?

Another related question with respect to overkill exhaust pipe volume:

If an exhaust extension pipe measures 4' x 3" (directly off the collector
of the header),
will that volume result in more backpressure due to more "atmosphere"
to overcome?

IE: Does the eddy flow, or reversion have anything to do with
the amount of atmospheric pressure residing in the pipe?

BTW: The above compared to a 2.5" extension pipe on a stock
350 CID small block spinning around 2500 RPM.

that question i am going to have to let somewhat else answer, i do not know enough about how much exhaust and air a 350 would flow at 2500. i'll let someone who knows n/a engines and how well the stock pieces and how a piece like that would flow answer this one, no need for myself to point someone in the wrong direction

Originally posted by MaxRaceSoftware
Will hot burnt exhaust gases "Burn a Second Time " ????

Why would you want hot burnt exhaust gases back inside your engine's cylinders ???

OldSStroker ..when i posted that , i wasn't replying to your previous post above mine ....but instead was just making a generalized statement towards all the other posts :)

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Larry, how much "extractor effect" do you think Cup cars get from placement of the flat exhaust outlet in the rocker area? It would seem the aero guys could create a low pressure area there and maybe actually measure the depression in the pipes in a wind tunnel. It might be significant at 190 mph.

on the Intake side its easier to calculate

Peak_RamAir_HP_Gain = MPH * MPH * Peak_HP * .000001205

Peak_RamAir_HP_Gain = 190 * 190 * 700HP * .000001205

Peak_RamAir_HP_Gain = 30.45 HP gain from RamAir

usually the same psi drop on exhaust side makes at least 1/3
gain of intake side
so approx = 10.15 HP at 190 mph would be a good guess ???

and also 10.15 extra free hp would increase fuel efficiency , BSFC, and gas mileage , and could mean difference between finishing or winning NASCAR race

If an exhaust extension pipe measures 4' x 3" (directly off the collector
of the header),
will that volume result in more backpressure due to more "atmosphere"
to overcome?
=======================

the wave passes right thru "medium" in extra length piece

but exhaust gas velocity would be so very slightly effected ,
you would have to have hi-tech equipment to measure

both the wave and the gas's velocity would be disipated
over time

when the collector "dumps" into the local atmosphere
you have the whole entire local atmosphere to "overcome"
it doesn't present a problem the way you are thinking the magnitude might be ?

Inert gases can't burn, they displace fresh intake charge

Do I get a cookie Larry/Bret/Jon?
=====================================


Zero_to_69

you get a Star :)

If 'stars' taste as good as cookies, I'm a happy man!

OK...pipe volume after the collector does not affect gas velocity
as much as I figured.

1.) For sake of argument, let's state that our 350 CID engine needs
800 CFM to make all its power.

If we were to reduce the extension pipe down to 1.5" diameter x 4' long,
the exhaust gas velocity would increase at a lower RPM correct?

On the other hand, 1.5" x 4' would not be sufficient for flow at
wide open throttle?

I don't recall the formula, but I'm thinking 800 CFM would need at
least 2.5" diameter to flow unrestricted?

Basically, all we would need for maximum exhaust pipe diameter
is what the engine requires at the intake (charge in = charge out).

2.) With respect to wave tuning, the length of the extension pipe is
more a factor (at full wave lengths, or harmonic lengths) than the
diameter?

So...a 4' extension would be resonant at xxx RPM, where 3' would
be resonant at a "certain" higher RPM (xxx + xx RPM) .

3.) Lastly, regarding the wave travel through the "medium":

If the acoustic wave, or pressuve wave length is longer than the
pipe length; the effect of the pulse peak (which is occuring somewhere
beyond the extension pipe exit) is not interfered with the atmosphere
residing in the extension pipe.

On the flip side, if the extension pipe was longer than the acoustic
wave, or pressure wave, the peak of the pulse is occuring within
the pipe and creating more of an impact at the exhaust port.

Please correct my errors :)

Hopefully I am starting to grasp these points. Keeping in mind the
pipe lengths and diameters would need to change size to be 100% efficient
across the RPM range in the above examples.

Originally posted by Zero_to_69


Basically, all we would need for maximum exhaust pipe diameter
is what the engine requires at the intake (charge in = charge out).



The volume out is considerably more than volume in due to combustion. True, there is considerably more pressure in the exhaust, but the inlet and exhaust flows are quite different.

Right...I forgot, fuel is not included in our breathing air :)

Originally posted by Zero_to_69
Right...

I believe OldSStroker was referring to the fact exhaust gases are heated. That being stated, the pipe diameter required, becomes less critical, as exhaust gases travel thru piping, with temp progressively decreasing. Now, if I'm off on a trangent here, well, ignore me.

Arnie,

Do you mean progressively reducing the pipe diameter as the
gas travels down the length of the exhaust?

how extreme can back pressure get? a while back i was wondering if you could use a really radical cam that closed fast (i forget the official tech term for that). by closing it faster and at the right spot you could make tremendous back pressure, enough to pull air in fast enough to creat boost. it would probably rob a little power as stress on the valves to cam. is that even possible, i was so confident when i first thought of it, hahaah.:p

Originally posted by Zero_to_69
Do you mean progressively reducing the pipe diameter as the
gas travels down the length of the exhaust?

Well yea, especially, as in smaller dia. post muffler. Same size dia. as gas travels, is not necessary to maintain equal flow. Temp much greater in header primaries, or even collector.

RE: Number77

IMHO, any amount of backpressure is extreme as it will degrade
power.

As for your idea about the backpressure "ramming" more air to
pressurize the cylinder, it ain't happening.

Higher pressure in the exhaust (backpressure) will prevent air
from entering the cylinder because there is a blockage in flow
in the exhaust port.

The bottleneck will cause the exhaust gas to remain in the cylinder
not allowing fresh intake charge to enter.

the valve design would also be a major issue, you can tell i mostly hang out in the lounge.:D

Originally posted by number77
you can tell i mostly hang out in the lounge.:D

LOL.

It's a lot more interesting around here. Lurk for a while and see.

I believe that backpressure can help hp/torque.

If you are overscavenging (allowing air/fuel to short-circuit though the chamber and out the exhaust during overlap) due to good low-lift flow, well tuned exhaust header, and well tuned intake runner, then backpressure can help hold the air/fuel in the chamber.

Granted, this seems to be a band-aid fix to a problem with the combination.

At any rate, nearly all of the modern high-performance street bikes have exhaust backpressure valves that adjust backpressure for the rpm/load. My theory is that they overscavenge in certain operating conditions, so the valve increases backpressure to hold the air in.

My '04 ZX-10R has a backpressure valve and makes 164 rwhp from 62 cid.

Mike

At any rate, nearly all of the modern high-performance street bikes have exhaust backpressure valves that adjust backpressure for the rpm/load. My theory is that they overscavenge in certain operating conditions, so the valve increases backpressure to hold the air in.
========================================

maybe or maybe not ?

"exhaust backpressure valves that adjust backpressure for the rpm/load."

its very possible this adjustible valve is still really causing a "negative" wave at exhaust/chamber during overlap period

the adjustible valve will change exhaust gas temps and densities to different values along exhaust pipe lengths

one way to determine would be p-v diagram or pressure transducer readouts.

that one Chrysler SS Hemi engine with 140.0+% Ve on my dyno was the worst "overscavenging" example i've ever seen

but the majority of its problem was just solved with new Cam change

and would have probably benefitted more with different cylinder head porting

all that change in overscavenging ..."WITHOUT" changing anything in exhaust headers

you always want the headers to work or scavenge,

but just because the SS Hemi had the completely wrong Cam in it and was combined with extremely good "Low-Lift" flowing ports
produced a very bad combination...it really wasn't fault of headers

If backpressure is helping power then the cam is in way wrong. At the most this could only used at lower speeds because it is not going to be there during the good part of the power band. Restricting the exhaust like restricting the intake is not going to ever help power in the fat part of the power band. This must be a bandaid from hell used in place of variable valve timing or something else but it is still interesting. The motorcycle market is famous for gimmicry that appears and then disappears like clockwork. If one of them does it then they will usually all do it so they can have that bell or whistle too. I know at least one of those systems is supposed to be worth some decent power if you remove it from what I've been told.

My '04 ZX-10R has a backpressure valve and makes 164 rwhp from 62 cid.
------------------------------------------------------------------------------------


This a stock ZX-10? Damnit boy, I needed a FULL array of mods on a 2001 Hayabusa to make that power, anyway I used to be heavily into sportbikes at one time. I rode a Gixxer 1000 when it was new with the valve in the exhaust, shortly after he put a Hindle race pipe on. With the valve in place in second gear roll on from a low rpm it would pick the front tire up in a lower RPM range than it would with the Hindle race system. Seemed to pull harder in the upper ranges and lose some lower RPM grunt with the Hindle on it. WIth no "flapper valve" as we called it. Hindle race system seemed to have the same size primary pipe, maybe even slightly smaller.



Anyway, anyone see my post on exhaust in this section....no one has really answered. I would really like to know some thoughts on it.


David

AKA FASTFATBOY

Originally posted by FASTFATBOY
This a stock ZX-10? Damnit boy, I needed a FULL array of mods on a 2001 Hayabusa to make that power. . .


Pretty sick, huh?

I don't understand how, though, since the 'Busa has a larger engine and much larger valves (34.5 mm Busa versus 31 mm ZX versus 29 mm GSXR). Maybe the 'Busa is "de-tuned" using smaller cams to get more mid-range and low. The ZX-10 torque peak occurs at 9,800 rpm.

Mike